Necessary, because if this were not true — if the amphibian species we’re talking about ARE known to be among the species whose populations don’t vary due to weather — then the author’s premise does not support the conclusion. If (A) were negated, natural weather variations wouldn’t be relevant to the amphibian species’ decline anymore.

Not necessary, because the negation of (B) helps the author’s argument. If the variations in populations that result from weather variation ARE always as large as the population declines that others attribute to pollution, then that’s even more reason to think natural weather variations could be the explanation for the population decline.

Not necessary, because the author never actually takes a position on what caused or didn’t cause the decline in the population. The author’s just saying that we can’t be sure it’s industrial pollution. But it might be industrial pollution, weather, or something else entirely that is the true cause.

Not necessary, because the author’s not committed to industrial pollution as the cause of the species’ population decline. The author doesn’t have to think industrial pollution has anything to do with the decline.

Not necessary, because the author never suggests any causal connection between pollution and weather variation.